Catheter based vascular interventions are becoming increasingly common in many of the vascular beds of the human body. For example, the treatment of obstructive plaque (e.g., stenosis) in coronary, peripheral, and cerebral arteries via angioplasty (with or without stents) has become a routine procedure. There remains a need, however, to improve the devices used in these procedures, to make them faster, easier, safer, and more viable, particularly in challenging anatomical situations.
The vast majority of catheter-based vascular interventions make use of a steerable guide wire to access the site of interest from a remote position outside the body. For example, in coronary interventions such as stent implantation, a steerable guide wire is advanced from the femoral artery access site into the various branches of coronary arteries and across the obstructive plaque. FIG. 5 illustrates the tip of a coronary guide wire accessing a coronary vessel with an obstructive plaque. After the guide wire is advanced past the stenosis, an interventional device such as a stent delivery balloon catheter (not shown) is advanced over the guide wire and through the stenosis. Thus, it is the guide wire that establishes the pathway for the interventional catheter that follows.
Steerability is an important performance characteristic for a steerable guide wire. Steerability generally refers to the ability to controllably rotate the distal tip of the guide wire to “point” the tip in the desired direction during the advancement procedure. Steerable guide wires typically have a “J” bend (for example, as seen in FIG. 5) imparted to the tip, either by the operator prior to the introduction into the body, or by the manufacturer. The ability to controllably orient this “J” bend allows the guide wire to be navigated into different branches of vessels and across the stenosis.
The ideal or optimum controllability of the tip of the guide wire is referred to as “1:1 torque response.” This term refers to the ability of the tip to rotate exactly in step with rotation of the proximal end of the guide wire. For example, if the proximal end of the guide wire is rotated through 90 degrees, the tip will ideally rotate through 90 degrees—hence a 1:1 response.
Several factors influence the steerability qualities of a steerable guide wire. These include torsional stiffness of the guide wire components, dimensions, torsional modulus, guide wire straightness, guide wire resilience (ability to bend without plastically deforming), lubricity, and cross-sectional configuration. Steerability is also impacted by the tortuosity of the vascular anatomy.
Another important characteristic of a steerable guide wire is its tensile strength/integrity. This term generally refers to the guide wire's ability to withstand tensile forces applied to it without breaking. For example, the tips of guide wires occasionally get lodged in the stenosis or elsewhere in the vasculature, and when this happens, it is important to be able to dislodge the tip by pulling on the proximal end of the guide wire. The design of prior art steerable guide wires has thus involved a balance or trade-off between optimizing flexibility and steerability while at the same time maintaining tensile integrity.
FIGS. 1A-1D illustrates a typical construction of a prior art steerable guide wire, such as those commonly used in coronary interventions. As seen in FIG. 1A, the guide wire generally includes three portions, a proximal portion, a mid-portion, and a distal tip portion. There are two main components in steerable guide wires, a core wire that extends from a proximal end to a distal end, and a coil that extends over the mid-portion and tip portion of the guide wire. Lubricious coatings such as PTFE and/or hydrophilic or hydrophobic materials may also be present over some or all portions of the guide wire.
The core wire component of the guide wire is typically fabricated of high tensile strength stainless steel wire; however other materials are also used, such as NITINOL, MP35N, or ELGILOY. The guide wire is relatively stiff in the proximal portion and becomes increasingly more flexible towards the distal end. The proximal portion is typically of the original wire diameter (e.g., 0.014 inches for a coronary guide wire). The mid-portion is made more flexible by grinding down the diameter of the core wire to one or more smaller dimensions (e.g., 0.005 to 0.010 inches).
The distal tip portion of the guide wire is made even more flexible by further grinding of the core wire to a smaller dimension (e.g., 0.002 to 0.003 inches). While grinding the core wire to these smaller diameters does impart flexibility to the core wire, it is typically still not flexible enough for the tip portion to be atraumatic to the vasculature. Therefore the dimension of the core wire in the tip region is reduced even further by stamping or rolling the round wire into a flat ribbon configuration. The ribbon structure is illustrated in FIGS. 2A and 2B, as well as Section C-C in FIG. 1D. As seen in FIGS. 1D, 2A, and 2B, the ribbon is formed integrally with the core wire. In an alternative method of manufacture, however, it is also known to attach a separately formed piece of ribbon to a distal end of the mid portion of the core wire.
The high degree of flexibility achieved by the ribbon configuration could theoretically be accomplished by grinding the core wire to a round dimension that gives the equivalent stiffness of the ribbon. Unfortunately, however, the cross-sectional area of such a round wire would be substantially less than the cross-sectional area of the ribbon configuration. Therefore, the tensile integrity of the core wire would be significantly lowered. In a commonly used steerable coronary guide wire, the dimensions of the ribbon structure of the tip portion are approximately 0.001 by 0.003 inches. Such dimensions in a high tensile strength stainless steel core wire yield a tip portion with a high degree of flexibility and a tensile strength of approximately 0.9 lbs, which is close to the minimum acceptable tensile strength integrity for the tip portion of the guide wire.
While the prior art guide wire described above has a tip portion with good flexibility and acceptable tensile integrity, it does have compromised steerability as a result of the ribbon structure in the tip portion. The ribbon portion is typically about 2 cm in length. Any time the tip portion is positioned in a tortuous region of the vasculature (such as illustrated in FIG. 5), the ribbon will naturally bend only in the direction perpendicular to the ribbon's widest dimension (e.g., out of the plane of the page as shown in FIG. 2B). For a ribbon structure, there are thus only two stable bending directions 180 degrees apart from each other.
If, in this anatomical setting, the guide wire is rotated in an effort to steer the tip, the tip will resist rotating. Torque or energy will be stored in the ribbon in the form of a twist in the proximal region of the ribbon, as well as in the core wire extending proximally from the ribbon. Continued rotation of the proximal end of the guide wire will cause enough torque to build up such that the tip portion will suddenly rotate or “whip” to its next stable orientation. This orientation is 180 degrees from the previous orientation. Therefore, the ability to rotate the tip to orientations between 0 and 180 degrees is hampered. Similarly, if the guide wire is further rotated, the tip portion will again resist rotating until enough torque is built up and then the tip will suddenly rotate an additional 180 degrees.
There is thus a need for a steerable guide wire that exhibits controllable steering of the tip even in anatomically challenging vasculature. Such a steerable guide wire should have excellent steerability, tip flexibility, as well as tensile integrity. Moreover, there is a further need for a guide wire that is able to be rotated at the proximal end without any “whipping” of the distal tip.